IUD Fell Out?

I'm on the Mirena IUD and I still get my period I'm not supposed to get my period I went to my doctor's and I'm going for an ultrasound to see if it's still inside me but what my question is why am I getting my period if I'm on the IUD

Update: my doctor told me that I'm not supposed to get my period. I was on it before this and I never got my period but I took that one out myself abd had thia one in since May once it was in me I never got a period until like August, it was light but over the next monrh or so it has become very heavy my doctor told me I'm not supposed to get my period while on the Mirena IUD, which is why she thinks it fell out she clipped the strings short so when she tried to feel for them she couldnt feel them so hence why I'm going for my ultrasound and if my Mirena IUD fell out I'm not going back on birth control my boyfriend and I are wanting a baby in the next several months we thought that if the IUD fell out or will be taken out what's the point of starting the Pill if him and I are going to be Trying To Concieve in the next few months that way if I was to go on the Pill and stop it, it would take awhile for my period to regulate so if either the IUD fell out or if it still in there I'm going to get it remove if still in me and just not go back on birth control. My doctor told me that the Mirena thins the lining of the uterus which makes me not get my period if there is no lining (blood) in my uterus I'm still ovulating but I'm not supposed to get my period. That is what my doctor told me